1. Field of the Invention
The present invention relates to an antenna apparatus, and more particularly to a miniaturized, high-performance antenna apparatus.
2. Description of the Background Art
Yagi antenna is generally used as an antenna receiving electric waves for television broadcasting in UHF (Ultra High Frequency) band or VHF (Very High Frequency) band. Yagi antenna includes one radiator, at least one reflector, and at least one director.
Although Yagi antenna is characterized by its excellent directivity, it is increased in size when the characteristics such as gain or front-to-back ratio (F/B ratio) are to be improved. Therefore, a variety of miniature and high-performance antennas have been proposed so far. For example, Japanese Patent Laying-Open No. 2003-8328 discloses a miniature antenna that allows the front-to-back ratio to be improved.
FIG. 20 shows an exemplary conventional miniature antenna. Referring to FIG. 20, an antenna apparatus 100 has a configuration such that a housing 102 contains radiators 103, 104 for UHF-band antenna, and a VHF-band antenna 106. VHF-band antenna 106 includes two radiators 106A and two radiators 106B.
Antenna apparatus 100 further includes a director 116 and a reflector 118. Radiators 103, 104, director 116, and reflector 118 constitute a UHF-band antenna. It is noted that a feed point of each of radiators 103, 104 is connected to a 2-way divider 122, and a feed point of each of radiators 106A, 106B is connected to a 2-way divider 112.
The tip end of radiator 103 is bent toward the director 116 side, and the tip end of radiator 104 is bent toward the reflector 118 side. In this way, radiators 103, 104 have bending portions to allow electric waves at a desired center frequency to be received and also allow miniaturization. Radiator 103 has a straight portion having a length of about 80 mm and a bending portion having a length of about 20 mm. Furthermore, radiator 104 has a straight portion having a length of about 110 mm and a bending portion having a length of about 40 mm. The distance between two radiators 103 is about 15 mm, and the distance between two radiators 104 is about 20 mm. The distance between radiator 103 and radiator 104 is about 90 mm.
The dimensions of the other parts of the UHF-band antenna will be described. The length of reflector 118 is about 300 mm. The distance from the most protrudent portion of reflector 118 to radiator 104 is about 40 mm. Furthermore, the distance between radiator 103 and director 116 is about 25 mm.
In antenna apparatus 100 shown in FIG. 20, the number of directors needs to be increased in order to improve the gain of the UHF-band antenna. However, if the number of directors is increased, the size of antenna apparatus 100 increases along the direction of travel of a signal S1 by electric waves. In addition, the area of reflector 118 needs to be increased in order to improve the front-to-back ratio of the UHF-band antenna. However, if the area of reflector 118 is increased, the size of antenna apparatus 100 is also increased.
As described above, the conventional antenna apparatus inevitably increases in size if its characteristics are intended to be improved. In other words, it is difficult to miniaturize the conventional antenna while its performance is maintained.